For many years it was quite common for service providers to utilize traditional Time Division Multiplexing (TDM) networks that employ TDM switches to carry information, such as voice signals, across the TDM network. In a typical arrangement, voice signals from a conversation originating at a telephone are converted to digital information and placed into TDM frames. The TDM frames travel through the TDM network via a series of TDM switches to a destination endpoint. At the destination endpoint, the voice information is extracted from the TDM frames and converted back to voice signals which are played by a, e.g., telephone at the destination of the conversation.
Nowadays, packet-based networks are increasingly being used to transport information contained in TDM frames over portions of TDM networks. The attractiveness of packet-based networks relates to their lower cost as well as their versatility.
Since much of the current-day communications networks have been built on a TDM-based architecture, many service providers have been replacing only portions of their TDM networks with packet-based networks in order to maintain backward compatibility with existing TDM equipment. Here, the service provider may replace selected TDM links with a packet-based network that comprises a combination of gateway devices and packet switches. Gateway devices are typically used because the format of TDM frames carried in the TDM network often differ from the format of packets carried in a packet network. The gateway devices are thus employed to “translate” between the TDM frames and the packets.
In a typical arrangement, TDM frames are forwarded to the gateway device which reformats information contained in the frames, such as voice signal information, into packets. These packets are then carried by packet switches in the packet network to another gateway device which receives the packets, reformats the information contained in the packets into one or more TDM frames, and transfers the frames over the TDM network. The gateway devices may also be configured to remove TDM frames corresponding to silence, and/or digitally compress the voice signals to enhance performance and attain better bandwidth efficiency.
Conventional gateway devices often “terminate” certain channels of a TDM frame because the channels carry information that is typically used locally between TDM switches. For example, it is not uncommon for gateways to terminate “channel 0” of E1 frames as this channel typically conveys synchronization information that is used locally by the TDM switches to synchronize the transfer of E1 frames across an E1 link.
One problem with terminating channels at a gateway is that data transmitted on the terminated channels are dropped and not propagated across the packet network. For example, as noted above, conventional gateway devices terminate “channel 0” of an E1 frame because the channel usually carries synchronization information. However, in accordance with the E1 standard, the channel may also carry data in bits known as “SA bits.” This SA bit data may include information that may be collected and used at various points in a TDM switch, such as operations and maintenance (OAM) data, performance data and the like. Since gateway devices normally terminate “channel 0” the data contained in the channel is lost.